theorem Th7:
  for p being QC-formula of A holds still_not-bound_in 'not' p =
  still_not-bound_in p
proof
  let p be QC-formula of A;
A1: 'not' p is negative;
  then the_argument_of 'not' p = p by QC_LANG1:def 24;
  hence thesis by A1,Th6;
end;
